[AS PER AMENDMENT 7/19/01: Volunteers in Rio de Janeiro, Haiti, and Trinidad and Tobago comprise Part I; volunteers in Sao Paulo and Peru comprise Part II.] Volunteers from Haiti, Brazil, [AS PER AMENDMENT 7/19/01: Peru] and Trinidad and Tobago are enrolled into 1 of 3 arms and followed for 18 months. Arm 1 volunteers receive ALVAC-HIV vCP1452 at 0, 1, 3, and 6 months. Arm 2 volunteers receive ALVAC-HIV vCP1452 on the same schedule as Arm 1 and receive HIV-1 MN rgp120 subunit simultaneously with the 3-month and 6-month vaccine doses. Arm 3 volunteers receive a placebo. Blood and urine samples are collected for immunologic assays, virologic determinations, pregnancy testing, and safety assessments. Risk behavior and social harms are assessed every 6 months during follow-up [AS PER AMENDMENT 7/19/01: Social harms are assessed every 3 months during follow-up and risk behavior every 6 months]. At all clinic visits volunteers receive counseling on avoidance of HIV infection and pregnancy. Participants are tested for HIV-1 every 3 [AS PER AMENDMENT 7/19/01: the following text has been deleted: to 6] months. Counseling and follow-up for any needed medical care are provided.